In the past decade, there has been great interest in organic electroluminescent devices, particularly conjugated polymer based light-emitting devices ("LED"s). Electroluminescence ("EL") combined with other unique properties of polymers, such as solution processibility, band gap tunability, and mechanical flexibility, make conjugated polymers excellent candidates for low cost large area display applications.
Among the most important limitations associated with "conventional" polymer light-emitting diodes is poor stability and so-called "shelf lifetime." The devices degrade even during storage. This is usually caused by the chemical reactivity of the low work function metal electrodes required for efficient electron injection and/or by the poor oxygen stability of most conjugated polymers. Recently there have been reports of new device configurations such as symmetrically configured AC light-emitting ("SCALE") devices and light-emitting electrochemical cells ("LEC"s). These devices modify the charge injection and/or transport characteristics such that the device operation is not sensitive to the electrode materials used. As a consequence, more stable metals such as aluminum or even gold can be used as electrodes, potentially improving the device operating stability and storage lifetimes.
To date, a variety of conjugated polymers and/or copolymers have been found to exhibit electroluminescent properties such that all the necessary colors needed for display applications are obtainable.
However, for most devices the color of the emitted light is fixed once the device is fabricated. Recently there has been great interest in developing color variable light-emitting devices, i.e., individual devices that can generate two or more colors of light. In color variable devices based on blends of polythiophene derivatives, different components in the blend emit different colors of light simultaneously, with the strength of each component varying with the applied voltage. Such devices can emit multiple colors of light; however, such devices have very limited control over brightness at a desired color. Color variable light-emitting electrochemical cells which emit two independent colors of light also have been developed. The two-color LECs offer an improved control over the color and brightness: the color is controlled by the polarity and the brightness is controlled by the magnitude of the driving voltage. However, due to the involvement of ionic species in the device operation, the response of the devices is intrinsically slow, making them clearly unsuitable for applications that require rapid switching of colors. More recently, multi-layer light emitting devices which generate two independent colors were achieved at liquid nitrogen temperature by inserting a blocking layer in between two different emitting polymer layers. The two colors can also be controlled by the polarity of the driving voltage. Such an approach improves the device response time, but it raises the device operating voltage due to the introduction of the charge blocking layer and retains the stability concerns of "conventional" polymer LEDs.
At present, most polymer-based LEDs can only be operated under forward DC bias, and require a low workfunction metal in the electron injecting contact. However, since low workfunction metals, such as calcium, are unstable against oxidation, such devices show very poor stability under ambient environment. Also, the conventional polymer LEDs generally only can emit one color of light, and it is not possible to tune the color of light once such LEDs have been fabricated.
The present invention thus is a further improvement upon the bipolar electroluminescent devices described in U.S. Pat. No. 5,663,573, which is incorporated herein by reference.
It is thus an object of the present invention to provide a color variable bipolar light emitting device that can be applied to a variety of display applications requiring a robust and reliable electroluminescent device.